Deterioration caused by the invasion of moisture beneath the exposed surfaces of articles used in outdoor environments is a well-known problem. This deterioration includes oxidative deterioration caused by reaction of water with the surfaces of reinforcing fibers used in these articles, as well as water-induced corrosion. In marine environments, for example, the problems associated with waterlogging are particularly compounded by the salinity of the environment. The presence of salt in such aqueous environments hastens the oxidative decomposition. In non-saline environments, for example in environments having high atmospheric humidity, water-resistant coatings are necessary to protect the structures and equipment surfaces from moisture-induced decomposition.
Articles affected by the deterioration described above include items having a surface exposed to high moisture or humidity. Examples of such articles include reinforced rods and cables, such as fiber optic or telecommunications cables. These telecommunications cables are often used in situations where they are buried underground or submerged in water over long periods. As such, protection from water damage is critical to the structural integrity of these cables and to the success of the functions they are intended to perform. A telecommunications cable, for example, may include a core comprising a glass rod that acts as a stiffening or reinforcing member. This rod contributes to the rigidity of the cable. When water penetrates to contact the core element of the cable, corrosion or chemical deterioration of the cable infrastructure may result.
In order to combat the problems associated with this waterlogging damage, several strategies have been devised in an attempt to provide water resistance to cables and other reinforced articles, and to protect their sensitive inner surfaces from contact with water or water vapor present in the surrounding environment. These techniques for making water-repellent articles have included wrapping the articles in a protective sheathing material; or sealing the surface to be protected. Sealing techniques may include chemically manipulating the surface layer of the article to render it resistant to water-absorption, or applying a repellent coating.
The technique of covering the surface with a protective sheathing material is conventional. It includes for example, using a wrap or tape made of an impervious polymer with water-blocking ability, or treating the wrapping material with an emulsion or solution of a water-blocking polymer. The sheathing process does not require application of a chemical compound or treatment to the surface of the article, rather the protection is derived only from the coverage by the sheathing material.
Coatings used to repel water traditionally have been composed of substances that are both insoluble and impenetrable to water, and therefore presented a physical barrier to encroaching moisture. Such barrier coatings have included materials such as greases or gels. In the case of cables, for example, these coatings are applied by extrusion under pressure. There are however, certain drawbacks associated with this type of coating. Greases or gels are difficult to handle because of their slipperiness, and they contribute an unpleasant feel to the coated article. This is an important factor to be considered in the manufacturing process, particularly because it affects the ease of handling of the cable during splicing operations. Greases and gels also undergo changes in viscosity at low or high temperatures. These viscosity changes may affect the freeze/thaw performance and therefore the stability of the coating. Poor performance in these respects therefore affects the stable performance of the cables.
More recently, greaseless, water-resistant dry coatings have been devised which, of themselves, have some degree of water-absorbing capacity. This ability to absorb water allows the coating to absorb the moisture contacting the article, while preventing direct contact with the sensitive surfaces. The absorbent component in these dry waterblocking coatings is a dry, granulated superabsorbent polymer that swells and absorbs upon contact with water. The superabsorbent polymers are usually characterized in terms of their swell rate, swell capacity and gel strength. Traditional uses for these dry superabsorbent polymers have primarily included personal hygiene product articles, food packaging articles and chemical spill cleanup compositions, however recent experimentation has included using these dry polymers to form coatings for other articles such as reinforced cables. For example, U.S. Pat. No. 5,689,601 to Hager, which is herein incorporated by reference, discloses a dry waterblocking coating for reinforcing fiber articles using a powdered or granulated water-soluble dry blocking ingredient encased in one or more thin layers of a sheathing polymer. This casing restricts the degree of water absorption that can be achieved by the granular polymer, and accordingly the swell capacity of this coating is limited.
The superabsorbent polymers traditionally used in dry waterblocking cable coating applications are dry, water-insoluble, granular polymers that are incorporated into various substrates such as yarn, binders and tape. The substrates typically also contain glass fibers as a form of reinforcement. However, as discussed above, the coatings formed with dry granulated blocking agents suffer the limitations of limited water swelling ability and swell rate as a necessary consequence of optimizing the gel strength. In the context of surface coatings, gel strength is defined as the ability to prevent water from wicking down the cable axis, particularly when the cables are used in aqueous environments where they are exposed to elevated water pressures. The swelling ability is directly related to the degree of cross-linking of the superabsorbent polymer. As the degree of cross-linking increases, so does the gel strength, but there is a related decrease in the swell rate and swell capacity of the polymer. The swell rate defines the amount of water that the coating absorbs over a fixed period of time. The swell capacity denotes the maximum amount of water or fluid absorbed by the coating, based on a measure of its dry weight. Consequently, coatings made of dry, granular, water-insoluble polymer are limited in their water-absorbing performance, as measured in terms of the swell rate and swell capacity.
Generally, coatings for reinforced fibers, strands and articles such as cables that are made from these fibrous materials are applied to the surface of the fibrous material and then cured before further processing, if any, occurs. The means of applying coatings, in general, differs depending on whether a fluid coating is used or whether a solid particulate coating is being applied. In the case of powdered coatings, the coating process using granulated water-blocking agents involves several time-consuming and labor- and equipment-intensive steps that are directly related to the use of a granulated polymer. These steps include the need for one or more treatments with a binding resin, and one or more applications of powdered resin at the powder-coating stations using apparatus such as a fluidized bed.
The means for applying fluid coatings may include flooding, or dipping the fibers or cables, for example, in a resin bath and then removing excess resin to form a consistent layer on the treated surface. In the case of strands, rovings or cables, the product is in the form of a continuous filament and therefore it can be passed through a stripper die to remove the excess resin. Alternatively, the coating may be sprayed onto the surface of the article. In order to form a coating layer that is thick enough to provide good coverage and protection from water penetration, the coating composition must be thick enough that it can adequately coat the article in one pass through the coating apparatus. In addition to thickness however, the composition must also have sufficient flowing ability to allow ready formation of a uniform coating on the surface of the article, and to prevent clogging of the coating apparatus, dye orifices or other machinery used to make polymer-coated fibrous articles. Traditionally in the art, in order to modify the viscosity of the fluid coating composition, dry particulate ingredients such as a flocculent polymer or starch have been used. The difficulty with such compositions is that the resulting composition after this solid ingredient is added is not homogenous. Rather, the composition contains varying levels of a particulate material, which makes handling difficult and also compromises the spreadability of the composition.
There exists in the art then, a need for a waterblocking coating composition for application to reinforced articles or reinforcing materials, which possesses excellent gel strength and wicking ability, as well as a high degree of water absorption and a concurrent, rapid swell rate. At the same time, a further need exists in the art for a coating composition that does not contain powdered polymer, and which, as a result, would not require a costly and labor intensive application process. Moreover, it is desired that such a coating composition exhibit good spreading and surface performance characteristics.